1. Technical Field
The present invention relates to a conveyance device that conveys print media with sprocket holes (engagement holes) such as tractor feed fanfold paper, and to a printing device having the conveyance device.
2. Related Art
Media conveyance devices of this type have a tractor that conveys print media in a specific conveyance direction by sequentially engaging engagement holes (“sprocket holes” below) formed in the print medium, and a conveyance roller disposed near the printhead of the printing device. The tractor and conveyance roller are connected by a drive power transfer mechanism such as a gear train or belt, and conveys the print medium in specific increments by controlling synchronously driving the tractor and conveyance roller. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2009-119574.
Because print media such as fanfold paper is unfolded and conveyed from a stack, the tractor and conveyance roller are driven at different conveyance speeds to apply tension to the print medium and remove slack and skewing. More specifically, the conveyance speed of the conveyance roller is faster than the conveyance speed of the tractor. Because the tractor engages the sprocket holes, the tractor holds the print medium with greater force than the conveyance roller, and the conveyance roller therefore slips against and applies tension to the print medium as described in Japanese Unexamined Patent Appl. Pub. JP-A-H04-159956.
In order to meet the need for high print quality, the conveyance distance of the conveyance roller must be controlled with high precision. To control conveyance with high precision, the coefficient of friction of the surface of the conveyance roller must be increased to reduce slipping against the print medium. However, if slipping of the conveyance roller is suppressed too much when the tractor and conveyance roller are connected by a drive power transfer mechanism as described above, conveyance differences due to the condition of the print medium and the conveyance precision of the tractor cannot be absorbed by the conveyance roller and accumulate, and the need for high precision conveyance cannot be met. Extreme tension may also be applied to the print medium when the tractor switches from forward to reverse conveyance, possibly resulting in the print medium tearing where the tractor engages the sprocket holes.